1967 NASCAR International Stock Car Racing Rule Book NASCAR Grand National Championship Circuit Section 20-7 -- Rear Axle

a. Rear axle ring and pinion may be any gear ratio.

b. Differentials optional. No quick change rear ends permitted.

c. Full floating rear axle compulsory, but must not alter tread width or general appearance. Limited slip differential permitted.

d. Differential oil coolers, approved by NASCAR, permitted.